How to convert an array object to a string in typescript constructor?

Recently I started learning Angular 2 with basic knowledge on typescript. So, I tried to work out code using constructor. But I'm getting type error in the VS Code editor and also unable to get the expected output in the browser. I am sharing the code below and attaching the screenshot too. It would be very helpful if anyone tell me how to convert the Object in the Array to a string in the typescript constructor?